There are a number of conventional processes for packaging integrated circuits. One approach which is commonly referred to as "flip chip" packaging generally contemplates forming solder bumps (or other suitable contacts) directly on I/O pads formed on an integrated circuit die. The die is then typically attached to a substrate such as printed circuit board or package substrate such that the die contacts directly connect to corresponding contacts on the substrate. That is, the die is placed on the substrate with the contact bumps facing corresponding contacts on the substrate. The solder contact bumps are then reflowed to electrically connect the die to the substrate.
When a flip chip is attached to the substrate, an air gap typically remains between flip chip and substrate. This gap is commonly filled with material that is flowed into the gap in liquid form and is then solidified. This material is generally a mixture of a resin and small silica spheres and is generally called underfill. The underfill material is typically applied in liquid form from a dispenser at one edge of a flip chip. The material then flows into the narrow gap and spreads across the flip chip until finally the entire area of the gap between flip chip and substrate is filled.
There are problems associated with underfill. For example, the operation of applying underfill must be repeated for each flip chip. Repeating such an operation many times adds to the cost of manufacture. Also, as the underfill material flows past solder bumps to fill the gap, separation of glass from resin may occur. The separation of silica spheres from the resin occurs as some silica spheres become trapped as they meet solder ball obstacles. The underfill material will develop streaks of high and low silica concentration. The silica may also separate from the resin by sinking to one side of the gap, thus creating a silica rich side and a resin rich side. This segregation of silica and resin alters the mechanical properties of the filled region and thereby negates the mechanical function of the underfill.
Therefore there is a need for a lower cost underfill application process and there is a need to reduce the amount of silica segregation that occurs.